1. Field of the Invention
The present invention relates to a method and a user equipment for performing random access.
2. Related Art
3rd generation partnership project (3GPP) long term evolution (LTE) is an improved version of a universal mobile telecommunication system (UMTS) and is introduced as the 3GPP release 8. The 3GPP LTE uses orthogonal frequency division multiple access (OFDMA) in a downlink, and uses single carrier-frequency division multiple access (SC-FDMA) in an uplink. The 3GPP LTE employs multiple input multiple output (MIMO) having up to four antennas. In recent years, there is an ongoing discussion on 3GPP LTE-advanced (LTE-A) that is an evolution of the 3GPP LTE.
In general, a terminal performs random access in order to access a network. Random access can be divided into contention-based random access and non-contention-based random access. The greatest difference between contention-based random access and non-contention-based random access is whether or not a random access preamble is dedicated to one terminal. In non-contention-based random access, a contention (or collision) with other terminals is not generated because a terminal uses a dedicated random access preamble designated thereto. Here, a contention means that two or more terminals attempt random access using the same random access preamble through the same resources. In contention-based random access, there is a possibility of a contention because a terminal uses a randomly selected random access preamble.
Random access is performed in order to perform uplink synchronization or to request the allocation of uplink radio resources. For example, after being initially powered on, a terminal can perform downlink synchronization and then perform random access in order to obtain uplink synchronization. For another example, in the state in which a Radio Resource Control (RRC) connection has not been established, a terminal can perform random access in order to have uplink radio resources for uplink transmission allocated thereto. For yet another example, a terminal may perform random access in order to initially access a target base station in a handover process.
Meanwhile, there are disadvantages in considering a cell having very small coverage of which needs has been growing recently, the conventional random access preamble is not proper.